In general, an output shaft of an electrical motor is journaled by bearings housed in a housing of the motor at the front and the rear thereof, and when the motor is applied as a drive source to an industrial machine such as a machine tool, a radial load and a thrust load are imposed on the output shaft thereof. Usually, the housing is formed by aluminum castings, and the front and the rear inner surfaces of this housing are often damaged, due to a direct pressure exerted thereon by the bearings. Therefore, a layer containing iron as a main component thereof, which is hard and thus is not easily damaged, is provided at the inner portion of the housing receiving such a pressure, to enable a hold structure for a bearing to withstand a radial load.
Nevertheless, an outside end face of either bearing is held by a member such as a C-shape ring when the bearings are incorporated in the housing, to retain the bearing so that it cannot move in the longitudinal direction of the motor. For this purpose, a groove is provided in the layer so that the member is retained in the inner portion of the housing, i.e., in the layer. The groove receives a thrust load acting on the output shaft through the bearings and the member, and therefore, is made of a ductile material containing iron as a main component thereof, such as FCD (Nodular Graphite Cast Iron) according to the JIS (Japanese Industrial Standard), to prevent damage.
On the other hand, after a central housing integrated with a stator is combined with a front or rear housing provided with the above-mentioned layer containing iron as a main component thereof at the inner portion thereof, an inner surface of the layer of a ductile material, which holds an outer ring of the bearing, is finish-machined to enhance the retain accuracy of the bearing, and a burr is often generated at the end point of the layer when machined, because the layer to be machined is made of a ductile material. When the burr is removed, a problem arises in that it sometimes drops into a stator coil winding, or that it prevents the incorporation of the bearings and a rotor.